Coal-based integrated gasification combined cycle plant (IGCC) technology enables production of electricity with a gas turbine utilizing a fuel that is rich in hydrogen and CO, and has a very limited amount of CO2. Combustion of the fuel requires an oxidizing source such as air, which contains nitrogen (N2). As a result, a by-product in exhaust gas stemming from hydrogen-containing fuel combustion is a significant amount of NOx and certain amounts of CO and SO2. During start up of an IGCC plant and also during the downtime of a gasifier, the IGCC plant is operating by using natural gas fuel. As a result, the combustion exhaust contains NOx, CO and hydrocarbons. NOx in the exhaust gas may be reduced by using selective catalytic reduction (SCR) systems along with low NOx combustors in the gas turbine. SCR is a very efficient NOx control device, especially when ammonia is used as a reducing agent.
In such systems, selective catalytic reduction is done by mixing the exhaust gas with anhydrous ammonia and passing the exhaust gas over a suitable reduction catalyst at temperatures between 150-550° C. prior to being released into the atmosphere. The ammonia is not a natural part of the combustion exhaust stream, but rather is injected into the exhaust stream upstream of the catalyst element for the specific purpose of supporting one or more of the following reduction reactions:4NH3+4NO+O2→4N2+6H2O  (1)4NH3+2NO+2NO2→4N2+6H2O  (2)8NH3+6NO2→7N2+12H2O  (3)2NH3+NO+NO2→2N2+3H2O  (4)
Reducing agents other than ammonia, such as for example hydrazine, methyl hydrazine, monomethyl amine, and urea, or mixtures thereof, or mixtures thereof with ammonia, may also be employed.
As mentioned above, it is also well known that IGCC exhaust includes elevated amounts of SO2. Commercial processes such as the Rectisol® process (licensed by both Linde AG and Lurgi AG), Selexol (now licensed by UOP LLC), and the like, can remove more than 97% of the sulfur from the subject gas. Still, the concentration of sulfur can be up to 20 ppm. Taking into consideration dilution with nitrogen, the concentration of SO2 in IGCC gas turbine exhaust can be at the level of 5 to 10 ppm. After CO2 sequestration and the burning of H2-fuel, concentrations of H2O in the exhaust can be as high as 20-25 vol. % and oxygen content can reach 10-18 vol. %. Under these conditions, developing a fuel flexible process to reduce NOx, CO, and hydrocarbon emissions is very challenging.
Critically, high sulfur content in the exhaust gas promotes the following reactions with excess of ammonia (ammonia slip), especially in the presence of high water concentrations:NH3+SO2+H2O→½O2→NH4HSO4  (5)2NH3+SO2+H2O→½O2→(NH4)2SO4  (6)
Formation of these undesirable ammonia salts, especially ammonia bisulfates (hereinafter “ABS”), may lead to severe corrosion of the heat exchange elements downstream of SCR. Furthermore, the salts may cause secondary pollution of PM2.5, which is an aerosol or mist of ammonia salts.